Methods of forming a micronized peptide coated stent

ABSTRACT

A coating for an implantable device such as a stent is provided including micronized peptides. A method of making the same is also provided.

CROSS REFERENCE TO RELATED APPLICATION

This application is a divisional application of U.S. patent application Ser. No. 10/177,942, filed on Jun. 21, 2002, and which issued as U.S. Pat. No. 8,506,617 on Aug. 13, 2013.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to coatings for implantable devices, such as stents, containing a micronized peptide.

2. Description of the Background

Blood vessel occlusions are commonly treated by mechanically enhancing blood flow in the affected vessels, such as by employing a stent. Stents act as scaffoldings, functioning to physically hold open and, if desired, to expand the wall of the passageway. Typically stents are capable of being compressed, so that they can be inserted through small lumens via catheters, and then expanded to a larger diameter once they are at the desired location.

Stents are used not only for mechanical intervention but also as vehicles for providing biological therapy. Biological therapy can be achieved by medicating the stents. Medicated stents provide for the local administration of an active agent at the diseased site. Local delivery of an active agent is a preferred method of treatment because the agent is concentrated at a specific site and thus smaller total levels of medication can be administered in comparison to systemic dosages that often produce adverse or even toxic side effects for the patient.

One method of medicating a stent involves the use of a polymeric carrier coated onto the surface of the stent. A composition including a solvent, a polymer dissolved in the solvent, and an active agent dispersed in the blend is applied to the stent by immersing the stent in the composition or by spraying the composition onto the stent. The solvent is allowed to evaporate, leaving on the stent surfaces a coating of the polymer and the active agent impregnated in the polymer.

A potential problem with the above-described method of medicating a stent is that there can be solvent incompatibility among the components of the composition. For example, if the various components are combined to form a composition, and one of the components is immiscible in the solvent of the composition, the immiscible component will form an agglomeration. If the active agent is insoluble in the solvent used for the composition, for instance, the active agent can form crystalline masses in the wet coating and will remain clumped together in the dry coating. When the coated stent is immersed in the aqueous environment of the body, the active agent will quickly disperse into the blood stream. This result is undesirable in many circumstances because therapeutic treatment often requires a prolonged and sustained release of the active agent from the coating matrix.

In view of the foregoing, it is desirable to achieve a process that allows components with incompatible solubility profiles to be combined in a composition used to coat stents. For example, it is desirable to achieve a process that allows polycationic peptides to be combined with hydrophobic polymers. The present invention addresses such problems by providing a coating for implantable devices, and a method of making the coating.

SUMMARY OF THE INVENTION

In accordance with one aspect of the invention, a stent used for implantation at a selected region of a vessel is disclosed including a coating having a polymer and a micronized peptide, such as a peptide having from 5 to 20 L-arginine subunits, or analogs or derivatives thereof, contained in the polymer for the release of the peptide subsequent to the implantation of the stent. In one embodiment, the peptide has an average particle size of less than 2 microns. In another embodiment, the solubility profile of the peptide is incompatible with the solubility profile of the polymer.

In accordance with another aspect of the invention, a method of coating a stent is disclosed including applying a composition to a stent, the composition including a micronized peptide and a fluid, and essentially removing the fluid from the composition on the stent to form a coating. The micronized peptide can be a peptide consisting of from 5 to 20 L-arginine subunits, or analogs or derivatives thereof. In one embodiment, the peptide is micronized by a process including providing a solution containing the peptide, atomizing the solution to produce an aerosol and directing the aerosol into a supercritical fluid to produce micronized particles. In another embodiment, the peptide is micronized by a process including providing a solution containing the peptide, atomizing the solution to produce an aerosol and directing the aerosol into a liquid gas to produce micronized particles.

DETAILED DESCRIPTION

For ease of discussion, the coatings and methods detailed herein will be described with reference to a coating for a stent, e.g., self-expandable or balloon expandable type. However, the device or prosthesis coated in accordance with embodiments of the present invention may be any suitable medical substrate that can be implanted in a human or veterinary patient. Examples of such implantable devices include stent-grafts, grafts (e.g., aortic grafts), artificial heart valves, cerebrospinal fluid shunts, pacemaker electrodes, and endocardial leads (e.g., FINELINE and ENDOTAK, available from Guidant Corporation). The underlying structure of the device can be of virtually any design. The device can be made of a metallic material or an alloy such as, but not limited to, cobalt chromium alloy (ELGILOY), stainless steel (316L), high nitrogen stainless steel, e.g., BIODUR 108, cobalt chrome alloy L-605, “MP35N,” “MP20N,” ELASTINITE (Nitinol), tantalum, nickel-titanium alloy, platinum-iridium alloy, gold, magnesium, or combinations thereof. “MP35N” and “MP20N” are trade names for alloys of cobalt, nickel, chromium and molybdenum available from Standard Press Steel Co., Jenkintown, Pa. “MP35N” consists of 35% cobalt, 35% nickel, 20% chromium, and 10% molybdenum. “MP20N” consists of 50% cobalt, 20% nickel, 20% chromium, and 10% molybdenum. Devices made from bioabsorbable or biostable polymers could also be used with the embodiments of the present invention.

Method of Coating a Stent

In an embodiment of the present invention, a method of coating a stent is provided, including combining a micronized active agent with a polymer and a solvent to form a composition, applying the composition to a stent, and removing the solvent from the composition on the stent to form a coating. By using active agent particles with reduced sizes, this method can be used to combine an active agent with a polymer to form a coating composition even though the active agent has a solubility profile that is not compatible with the solubility profile of the polymer. In other words, the method can be used to combine the active agent with a polymer even though the active agent is immiscible in the solvents that dissolve the polymer.

In a particular example, it has been difficult to incorporate L-arginine or its oligomers into a conventional polymer coating. L-arginine, commonly abbreviated as “R” or “Arg,” also known as 2-amino-5-guanidinovaleric acid, has a formula NH═C(NH₂)—NH—(CH₂)₃—CH(NH₂)—COOH. L-arginine is an amino acid. Due to the presence of a strong basic guanidinium group, —NH—C(NH₂)═NH, carrying a partially uncompensated positive charge, L-arginine, and its polymers and/or oligomers are highly cationic. For example, the heptamer of L-arginine has a pK_(a) of 13.2.

Polymers and/or oligomers of L-arginine that can be used are referred to herein as “PArg.” PArg are polycationic peptides comprising a plurality of repeating monomeric amino acid units and have a general formula H—[NH—CHX—CO]_(p)—OH, where “p” can be within a range of 5 and 1,000, typically, within a range of between 5 and 20. For example, a heptamer (designated R7) or a nonamer (R9), having p=7 and p=9, respectively, can be used. In the formula of PArg, “X” is 1-guanidinopropyl radical having the structure —CH₂—CH₂—CH₂—NH—C(NH₂)═NH. The terms “polymers and/or oligomers of L-arginine” and “PArg” are intended to include L-arginine in both its polymeric and oligomeric form.

In addition to PArg, other polycationic peptides can be alternatively used. Examples of such alternative polycationic peptides include poly(L-arginine), poly(D-arginine), poly(L-lysine), poly(D-lysine), poly(δ-guanidino-α-aminobutyric acid), racemic mixtures of poly(L-arginine) and poly(D-arginine), and chitosan.

As discussed above, L-arginine is highly cationic due to the strong basicity of the guanidinium group of L-arginine. This high degree of polarity causes L-arginine and its polymers and/or oligomers to be generally insoluble in most organic solvents, leaving water as one of the few solvent alternatives. However, most coating polymers (e.g., ethylene vinyl alcohol copolymer) are soluble in organic solvents, but not in water. Accordingly, the solvent profile of PArg is incompatible with many polymers, and therefore it is difficult to incorporate PArg into stent coatings using conventional coating techniques. By using particles of PArg with reduced sizes, the method of the present invention can be used to combine PArg with a polymer to form a coating composition even though the PArg has a solubility profile that is not compatible with the solubility profile of the polymer.

“Micronized” refers to particles that have been reduced in size to only a few microns in diameter, or a solution which has been atomized so that the liquid droplets of the solution are only a few microns in diameter.

“Organic solvent” refers to an organic solvent or a solvent mixture having at least one organic component. The solvent mixture can also include mixtures of water with miscible organic solvents.

“Solubility profile” refers to the general solubility characteristics of a particular compound or mixture of compounds including the solvents that can be used to dissolve the compound or mixture of compounds, and the physical variables such as pH and temperature that can be used to increase or decrease the solubility of the compound or mixture of compounds.

“Hydrophilic” refers to a component is that is water-soluble, or has an affinity for absorbing water (e.g., a hydrogel polymer) or tends to combine with water.

“Water-soluble” refers to components which have a greater solubility in water than in organic solvents and are commonly highly soluble in water. Although certain regions or segments of the component may be hydrophobic, the component as a whole is capable of substantially dissolving in water or aqueous-based systems, or a highly polar solvent(s). Typical aqueous solubilities of hydrophilic components at 20° C. will be equal to or greater than 5 mg/ml, usually greater than 50 mg/ml, and often greater than 100 mg/ml.

“Hydrophobic” refers to a component that is water-insoluble, or does not have an affinity for absorbing water or does not tend to combine with water.

“Water-insoluble” refers to components which have a greater solubility in organic solvents than in water and are commonly insoluble or poorly soluble in water. Although certain regions or segments of the component may be hydrophilic, the component as a whole is capable of substantially dissolving in organic solvents. Typically, the solubility for such components in water at 20° C. will be below 5 mg/ml, usually below 1 mg/ml.

Once the micronized active agent has been combined with the polymer and the solvent to form a composition, the composition can be applied to a stent by any conventional method, such as by spraying the composition onto the device or immersing the device in the composition. The stent can optionally be partially pre-expanded prior to the application of the composition. For example, the stent can be radially expanded about 20% to about 60%, more narrowly about 27% to about 55%—the measurement being taken from the stent's inner diameter at an expanded position as compared to the inner diameter at the unexpanded position. The expansion of the stent, for increasing the interspace between the stent struts during the application of the composition, can further prevent “web” formation between the stent struts.

Operations such as wiping, centrifugation, or other web clearing acts can be performed to achieve a more uniform coating. Briefly, wiping refers to the physical removal of excess coating from the surface of the stent; and centrifugation refers to rapid rotation of the stent about an axis of rotation. The excess coating can also be vacuumed off of the surface of the stent.

After the composition is applied to the stent, the solvent is removed from the composition on the stent by allowing the solvent to evaporate. The evaporation can be induced by heating the stent at a predetermined temperature for a predetermined period of time. For example, the stent can be heated at a temperature of about 80° C. for about 1 hour to about 24 hours. The heating can be conducted under a vacuum condition. It is understood that essentially all of the solvent will be removed from the composition but traces or residues can remain blended with the other ingredients.

The particular thickness of the coating is based on the type of procedure for which the stent is employed and the amount of active agents that is desired to be delivered. By way of example and not limitation, the coating can have a thickness of 2 microns to about 15 microns. Applying a plurality of reservoir coating layers, containing the same or different active agent, onto the stent can further increase the amount of the active agent to be carried by the stent.

Methods of Micronizing the Active Agent

Before the active agent is combined with the polymer and solvent, the active agent should be micronized to reduce the size of active agent particles. For example, particles of active agent can be micronized by wet milling or dry milling. Additionally, a solution of the active agent can be atomized to produce an aerosol, and the aerosol can be directed into a supercritical fluid or a liquid gas in order to produce micronized particles. In one embodiment, the average particle size is less than 2 microns.

“Particle” refers to solid particles. “Average particle size” refers to the average of the largest dimension of the particles. Preferably, the size distribution of the particle sizes will be substantially uniform. For example, the micronized active agent will have particles with a size distribution where a substantial majority (e.g., at least 90%) of the particles have a particle size less than about 2 microns. Additionally, it is preferred that the particle size distribution does not have an excess amount of particles with very small particle sizes (e.g., less than 0.3 microns). The average particle size can be determined by a number of conventional particle size analyzers such as a LA-300 Particle Size Analyzer (Horiba Laboratory Products, Irvine, Calif.).

Dry Milling

Several different types of mills can be used to micronize the active agents. For example, some dry mills are capable of grinding the active agent into ultrafine particles through mechanical impact and/or attrition, such as high-speed stirring mills and impact mills. Representative examples of dry mills are cylinder-type mills such as a rotating ball mill, vibrating ball mills, and hammer mills. A particle size analyzer can be used during the micronization process to ensure that the active agent particles are being ground to the desired average particle size.

Another representative example of a dry mill for the present process is a jet mill which is a size reduction machine in which particles to be ground are accelerated in a stream of gas, (e.g., compressed air) and micronized in a grinding chamber by their impact against each other or against a stationary surface in the grinding chamber. Different types of jet mills can be categorized by their particular mode of operation. Jet mills, for instance, may be distinguished by the location of feed particles with respect to incoming air. Micronization can be performed by any number of commercially available jet mills such as the Micron-Master® (The Jet Pulverizer Company, Moorestown, N.J.). The Micron-Master® is a continuous operating pulverizer with capacities of ½ to 4000 lbs/hr and can grind crystalline or friable materials, in a dry state, finely and uniformly to an average particle size of about 0.25 microns to about 5 microns. In addition, the Micron-Master® can produce a substantially uniform homogeneous random blend of micronized particles from a mix of two or more active agents.

Wet Milling

The active agent can also be micronized by conventional wet milling techniques. Wet milling techniques utilize an aqueous solution or an organic solvent during the process, and can use mills similar to those used in dry milling techniques. Suitable mills for wet milling can include hammer mills, impact mills (where particle size reduction/control is achieved by impact of the active agent particles with metal blades and retained by an appropriately sized screen), and sand mills (where particle size control/reduction is achieved by contact of the active agent with hard media such as sand or zirconia beads). Another type of conventional wet milling equipment that can be used for the procedure includes an ultrasonic homogenizer (Cole-Parmer Instrument Company, Vernon Hills, Ill.). As an example, a wet milling procedure can be carried out in an aluminum vessel filled with 3 mm-zirconia grinding balls (YTZ® grinding media, Tosoh Co., Japan), and a polymer and organic solvent.

The wet milling procedure can also be used in addition to the dry milling procedure. For example, a previously dry milled active agent can be subjected to a wet milling that will result in a further reduction of the average particle size initially imparted by the dry milling procedure.

Exposure to Liquid Gasses

The active agent can also be micronized by exposing the active agent to a liquid gas. In particular, the active agent can be dissolved in solution, and atomized to produce an aerosol by a conventional spraying apparatus. The aerosol can then be directed into a liquid gas which quickly freezes the liquid droplets of active agent solution to form solid particles. Because the liquid droplets are quickly frozen, the droplets do not have sufficient time to aggregate into larger droplets.

Representative examples of liquid gasses that can be used include nitrogen, argon, helium, hydrogen and oxygen.

If the active agent is initially dissolved in an aqueous solution, after the droplets are frozen, the water in the particles can be removed. For instance, frozen active agent particles can be freeze-dried by using conventional methods, or the water from the active agent particles can be extracted by water miscible solvents such as acetone or tetrahydrofuran.

Depending on the process parameters and the chemical characteristics of the active agent (e.g., peptide) and other components of the composition, the active agent may not be stable in solution. Therefore, it may be necessary to add a stabilizer to the active agent solution such as a surfactant (e.g., sodium dodecyl sulfate), a carbohydrate (e.g., a polysaccharide such as heparin), or a water-soluble salt such as sodium chloride.

Exposure to Supercritical Fluids

The active agent particles can be micronized by a process that utilizes supercritical fluids. Typically this process includes first dissolving an active agent in an aqueous solution, which is then mixed under pressure with a supercritical fluid to form an emulsion. Next, the mixture is directed through a nebulizer which atomizes the mixture through rapid decompression to form an aerosol of frozen particles. The micronized particles are then directed through a drying chamber in order to remove the water from the particles, after which the particles are collected. The foregoing process can be performed by using conventional micronizing equipment, such as the Temco Instruments Model BD-200 Bubble Dryer™ (Temco Instruments, Tulsa, Okla.).

As used herein the term “supercritical fluid” should be considered to encompass near-supercritical fluids, i.e., highly compressed fluids that are below the critical temperature point, yet exhibit many of the same qualities of true supercritical fluids, such as high solvating power and compressibility. Representative examples of supercritical fluids include carbon dioxide, sulfur hexafluoride, chlorofluorocarbons, fluorocarbons, nitrous oxide, xenon, propane, n-pentane, ethanol, nitrogen, and water. Depending on the active agent, the solubility of the supercritical fluid may be less than is desirable, and therefore the supercritical fluids can be a mixture of the foregoing in order to increase the solubility. In one preferred embodiment, the supercritical fluid is carbon dioxide or mixtures of carbon dioxide with another gas such as fluoroform or ethanol. Carbon dioxide has a critical temperature of 31.3° C. and a critical pressure of 72.9 atmospheres (1072 psi), low chemical reactivity, physiological safety, and is relatively inexpensive.

During this process, depending on the process parameters and the chemical characteristics of the active agent (e.g., peptide) and other components of the composition, the active agent may not be stable in solution. Therefore, it may be necessary to add a stabilizer to the active agent solution such as a surfactant (e.g., sodium dodecyl sulfate), a carbohydrate (e.g., a polysaccharide such as heparin), or a water-soluble salt such as sodium chloride.

Embodiments of the Composition

The composition used to form the coating for the stent can include a solvent, a polymer dissolved in the solvent, and an active agent. Representative examples of polymers that can be used to coat a stent in accordance with the present invention include ethylene vinyl alcohol copolymer (commonly known by the generic name EVOH or by the trade name EVAL®); polybutylmethacrylate; poly(hydroxyvalerate); poly(L-lactic acid); polycaprolactone; poly(lactide-co-glycolide); poly(hydroxybutyrate); poly(hydroxybutyrate-co-valerate); polydioxanone; styrene-ethylene/propylene-styrene polymers (KRATON® G series, available from Kraton Polymers, Houston, Tex.); polyorthoester; polyanhydride; poly(glycolic acid); poly(D,L-lactic acid); poly(glycolic acid-co-trimethylene carbonate); polyphosphoester; polyphosphoester urethane; poly(trimethylene carbonate); poly(iminocarbonate); copoly(ether-esters) (e.g. PEO/PLA); polyalkylene oxalates; polyphosphazenes; biomolecules, such as fibrin, fibrinogen, cellulose, starch, collagen and hyaluronic acid; polyurethanes; silicones; polyesters; polyolefins; polyisobutylene and ethylene-alphaolefin copolymers; acrylic polymers and copolymers; vinyl halide polymers and copolymers, such as polyvinyl chloride; polyvinyl ethers, such as polyvinyl methyl ether; polyvinylidene halides, such as polyvinylidene fluoride and polyvinylidene chloride; polyacrylonitrile; polyvinyl ketones; polyvinyl aromatics, such as polystyrene; polyvinyl esters, such as polyvinyl acetate; copolymers of vinyl monomers with each other and olefins, such as ethylene-methyl methacrylate copolymers, acrylonitrile-styrene copolymers, ABS resins, and ethylene-vinyl acetate copolymers; polyamides, such as Nylon 66 and polycaprolactam; alkyd resins; polycarbonates; polyoxymethylenes; polyimides; polyethers; polyisocyanate; polyethylene glycol; polyvinyl pyrrolidone; poly(2-hydroxyethyl methacrylate); polyacrylic acid; epoxy resins; polyurethanes; rayon; rayon-triacetate; cellulose acetate; cellulose butyrate; cellulose acetate butyrate; cellophane; cellulose nitrate; cellulose propionate; cellulose ethers; and carboxymethyl cellulose.

In accordance with an embodiment of the invention, a hydrophobic component (e.g., polymer) is dissolved in an organic solvent. “Solvent” refers to a liquid substance or composition that is compatible with the target component and is capable of dissolving the component at the concentration desired in the composition. A representative example of a solvent for hydrophilic components includes water. Representative examples of solvents for hydrophobic components include chloroform, acetone, dimethylsulfoxide, propylene glycol methyl ether, iso-propylalcohol, tetrahydrofuran, dimethylformamide, dimethyl acetamide, benzene, toluene, xylene, hexane, cyclohexane, heptane, octane, nonane, decane, decalin, ethyl acetate, butyl acetate, cyclohexanone, cyclohexanol, 1,4-dioxane, isobutyl acetate, isopropyl acetate, butanol, diacetone alcohol, benzyl alcohol, 2-butanone, cyclohexanone, dioxane, methylene chloride, carbon tetrachloride, tetrachloroethylene, tetrachloroethane, chlorobenzene, 1,1,1-trichloroethane, formamide, hexafluoroisopropanol, 1,1,1-trifluoroethanol, and hexamethyl phosphoramide and a combination thereof.

In an embodiment of the present invention, a micronized active agent is added to a polymer and a solvent to form a composition for a stent coating. In another embodiment, a second active agent that has not been micronized can also be added to the composition. The active agent can be for inhibiting the activity of vascular smooth muscle cells. More specifically, the active agent can be aimed at inhibiting abnormal or inappropriate migration and/or proliferation of smooth muscle cells for the inhibition of restenosis. The active agent can also include any substance capable of exerting a therapeutic or prophylactic effect in the practice of the present invention. For example, the active agent can be for enhancing wound healing in a vascular site or improving the structural and elastic properties of the vascular site.

An example of a suitable group of active agents includes hydrophilic peptides, including oligopeptides and polypeptides, and proteins. For instance, the hydrophilic peptide could be a peptide having from 5 to 20 L-arginine subunits, or functional analogs or structural derivatives thereof. In a more particular example, the peptide can usefully be heptaarginine (R7). L-arginine and its oligomers are known to be therapeutically beneficial substances. One beneficial property of L-arginine and its oligomers is that L-arginine is known to be a precursor of endothelium derived nitric oxide (NO). NO is synthesized from L-arginine and its oligomers by the enzyme NO synthase, a homodimeric flavo-hemoprotein that catalyzes the 5-electron oxidation of L-arginine to produce NO and L-citrulline. Among other therapeutic properties, NO relaxes vascular smooth muscle and inhibits its proliferation.

“Peptide” refers to a compound made up of a single chain of D- or L-amino acids or a mixture of D- and L-amino acids joined by peptide bonds.

“Oligopeptide” refers to a peptide composed of less than 50 amino acid residues.

“Polypeptide” refers to a peptide composed of at least 50 amino acid residues.

“Protein” refers to a macromolecule composed of one or more polypeptide chains with one or more characteristic conformations.

Representative examples of other active agents include antiproliferative substances such as actinomycin D, or derivatives and analogs thereof (manufactured by Sigma-Aldrich 1001 West Saint Paul Avenue, Milwaukee, Wis. 53233; or COSMEGEN available from Merck). Synonyms of actinomycin D include dactinomycin, actinomycin IV, actinomycin I₁, actinomycin X₁, and actinomycin C₁. The active agent can also fall under the genus of antineoplastic, anti-inflammatory, antiplatelet, anticoagulant, antifibrin, antithrombin, antimitotic, antibiotic, antiallergic and antioxidant substances. Examples of such antineoplastics and/or antimitotics include paclitaxel (e.g. TAXOL® by Bristol-Myers Squibb Co., Stamford, Conn.), docetaxel (e.g. Taxotere®, from Aventis S.A., Frankfurt, Germany) methotrexate, azathioprine, vincristine, vinblastine, fluorouracil, doxorubicin hydrochloride (e.g. Adriamycin® from Pharmacia & Upjohn, Peapack N.J.), and mitomycin (e.g. Mutamycin® from Bristol-Myers Squibb Co., Stamford, Conn.). Examples of such antiplatelets, anticoagulants, antifibrin, and antithrombins include sodium heparin, low molecular weight heparins, heparinoids, hirudin, argatroban, forskolin, vapiprost, prostacyclin and prostacyclin analogues, dextran, D-phe-pro-arg-chloromethylketone (synthetic antithrombin), dipyridamole, glycoprotein IIb/IIIa platelet membrane receptor antagonist antibody, recombinant hirudin, and thrombin inhibitors such as Angiomax™ (bivalirudin, Biogen, Inc., Cambridge, Mass.). Examples of such cytostatic or antiproliferative agents include angiopeptin, angiotensin converting enzyme inhibitors such as captopril (e.g. Capoten® and Capozide® from Bristol-Myers Squibb Co., Stamford, Conn.), cilazapril or lisinopril (e.g. Prinivil® and Prinzide® from Merck & Co., Inc., Whitehouse Station, N.J.); calcium channel blockers (such as nifedipine), colchicine, fibroblast growth factor (FGF) antagonists, fish oil (omega 3-fatty acid), histamine antagonists, lovastatin (an inhibitor of HMG-CoA reductase, a cholesterol lowering drug, brand name Mevacor® from Merck & Co., Inc., Whitehouse Station, N.J.), monoclonal antibodies (such as those specific for Platelet-Derived Growth Factor (PDGF) receptors), nitroprusside, phosphodiesterase inhibitors, prostaglandin inhibitors, suramin, serotonin blockers, steroids, thioprotease inhibitors, triazolopyrimidine (a PDGF antagonist), and nitric oxide. An example of an antiallergic agent is pemirolast potassium. Other therapeutic substances or agents which may be appropriate include alpha-interferon, genetically engineered epithelial cells, retinoic acid, suramin, asiaticoside, hyaluronan, heparin sulfate, heparin having a hydrophobic counterion, mannose-6-phosphate, superoxide dismutase rapamycin, rapamycin analogs and derivatives, and dexamethasone. The foregoing substances are listed by way of example and are not meant to be limiting. Other active agents which are currently available or that may be developed in the future are equally applicable.

The dosage or concentration of the active agent required to produce a favorable therapeutic effect should be less than the level at which the active agent produces toxic effects and greater than the level at which non-therapeutic results are obtained. The dosage or concentration of the active agent required to inhibit the desired cellular activity of the vascular region can depend upon factors such as the particular circumstances of the patient; the nature of the trauma; the nature of the therapy desired; the time over which the ingredient administered resides at the vascular site; and if other active agents are employed, the nature and type of the substance or combination of substances. Therapeutic effective dosages can be determined empirically, for example by infusing vessels from suitable animal model systems and using immunohistochemical, fluorescent or electron microscopy methods to detect the agent and its effects, or by conducting suitable in vitro studies. Standard pharmacological test procedures to determine dosages are understood by one of ordinary skill in the art.

Preparation of the Composition

The composition can be prepared by conventional methods wherein all components are combined, then blended. More particularly, a predetermined amount of a polymer can be added to a predetermined amount of a solvent or a combination of solvents. The mixture can be prepared in ambient pressure and under anhydrous atmosphere. Heating and stirring and/or mixing can be employed to effect dissolution of the polymer into the solvent.

Next, sufficient amounts of a micronized active agent is dispersed in the blended mixture of the polymer and the solvent. “Dispersed” refers to the particles being substantially present as individual particles, not agglomerates or flocs. In certain polymer-solvent blends, certain micronized active agents will disperse with ordinary mixing. Otherwise the micronized active agent can be dispersed in the composition by high shear processes such as using a rotor stator mixer or ultrasonication, which are well known to one of ordinary skill in the art. Biocompatible dispersing agents in the form of surfactants, emulsifiers, or stabilizers may also be added to the blend to assist in particle dispersion.

If a second active agent is added to the composition, and is not micronized, the second active agent is usefully put into solution in the blended composition. If the active agent is not completely soluble in the composition, operations including mixing, stirring, and/or agitation can be employed to effect homogeneity of the residues. The mixing of the active agent can be conducted in an anhydrous atmosphere, at ambient pressure, and at room temperature.

It should be noted that although the methods of the present invention are particularly useful for forming pharmaceutical compositions where the hydrophilic component is a hydrophilic active agent and the hydrophobic component is a hydrophobic polymer, the methods may be applied more broadly to form compositions including hydrophobic active agents and hydrophilic polymers.

Optional Coating Layers

In one embodiment, an optional primer layer can be formed prior to the reservoir coating. “Reservoir coating” refers to the coating which includes an active agent. The primer can increase the retention of the reservoir coating on the surface of the stent, particularly metallic surfaces such as stainless steel. The primer layer can act as an intermediary adhesive tie layer between the surface of the device and a reservoir coating, allowing for the quantity of the active agent to be increased in the reservoir coating. To form an optional primer layer on the surface of the stent, a composition that is free from active agents is applied to the surface of the stent.

A representative example of a polymer to be used as a primer layer is ethylene vinyl alcohol copolymer which adheres very well to metallic surfaces, particularly stainless steel. Accordingly, the copolymer provides for a strong adhesive tie between the reservoir coating and the surface of the implantable device. Another representative example of a polymer that can be used in a primer layer is polybutylmethacrylate.

With the use of thermoplastic polymers such as, but not limited to, ethylene vinyl alcohol copolymer, polycaprolactone, poly(lactide-co-glycolide), and poly(hydroxybutyrate), the deposited primer composition should be exposed to a heat treatment at a temperature range greater than about the glass transition temperature (T_(g)) and less than about the melting temperature (T_(m)) of the selected polymer. Unexpected results have been discovered with treatment of the composition under this temperature range, specifically strong adhesion or bonding of the coating to the metallic surface of the implantable device. The device should be exposed to the heat treatment for any suitable duration of time that will allow for the formation of the primer layer on the surface of the implantable device and for the evaporation of the solvent employed. By way of example and not limitation, the optional primer layer can have a thickness of about 0.01 microns to about 1 micron. The application of the reservoir coating should be performed subsequent to the drying of the optional primer layer.

In another embodiment, an optional diffusion barrier can be formed over a reservoir coating to reduce the rate at which the active agent is released from the coated stent. A composition, free from any active agents, can be applied to a selected portion of the reservoir coating subsequent to the drying of the reservoir coating. The diffusion barrier may be composed of a different polymer from that used in the reservoir coating or the same material. The diffusion barrier can have a thickness of about 0.2 microns to about 10 microns. It is understood by one of ordinary skill in the art that the thickness of the diffusion barrier is based on factors such as the type of stent, the type of procedure for which the stent is employed, and the rate of release that is desired.

Method of Use

In accordance with embodiments of the above-described method, an active agent can be applied to an implantable device or prosthesis, e.g., a stent, retained on the stent during delivery and expansion of the stent, and released at a desired rate and for a predetermined duration of time at the site of implantation. A stent having the above-described coating is useful for a variety of medical procedures, including, by way of example, treatment of obstructions caused by tumors in bile ducts, esophagus, trachea/bronchi and other biological passageways. A stent having the above-described coating is particularly useful for treating occluded regions of blood vessels caused by abnormal or inappropriate migration and proliferation of smooth muscle cells, thrombosis, and restenosis. Stents may be placed in a wide array of blood vessels, both arteries and veins. Representative examples of sites include the iliac, renal, and coronary arteries.

Briefly, an angiogram is first performed to determine the appropriate positioning for stent therapy. An angiogram is typically accomplished by injecting a radiopaque contrasting agent through a catheter inserted into an artery or vein as an x-ray is taken. A guidewire is then advanced through the lesion or proposed site of treatment. Over the guidewire is passed a delivery catheter which allows a stent in its collapsed configuration to be inserted into the passageway. The delivery catheter is inserted either percutaneously or by surgery into the femoral artery, brachial artery, femoral vein, or brachial vein, and advanced into the appropriate blood vessel by steering the catheter through the vascular system under fluoroscopic guidance. A stent having the above-described coating may then be expanded at the desired area of treatment. A post-insertion angiogram may also be utilized to confirm appropriate positioning.

EXAMPLES

The embodiments of the present invention will be illustrated by the following set forth examples. All parameters and data are not to be construed to unduly limit the scope of the embodiments of the invention.

Example 1

0.4 grams of dry heptaarginine (R7) is combined with 0.4 grams of polybutylmethacrylate (PBMA) and 4 grams of toluene in a cylindrical, 20 cm³ stainless steel mill. 10 cm³ of 1 mm zirconia beads (YTZ® grinding media, Tosoh Co., Japan) are added to the mill. The mill is sealed and shaken to reduce the size of the R7 particles. During the process, the average particle size of the R7 particles is monitored by using a LA-300 Particle Size Analyzer (Horiba Laboratory Products, Irvine, Calif.). After the R7 particles have been reduced to the desired average particle size, a solution of 0.8 grams of PBMA and 8 grams of toluene is added to the mill, and the mill is additionally shaken to homogenize the mixture. The mixture is then poured through a 0.3 mm sieve to remove the zirconia beads. For spray application, the viscosity of the mixture can be further reduced by adding toluene. The composition that results from the foregoing process can then be applied to a stent by using an EFD 780S spray coater (manufactured by EFD Inc., East Providence, R.I.).

Example 2

0.4 grams of dry R7 is combined with 0.4 grams of cellulose acetate butyrate (CAB), 3.2 grams of toluene and 0.8 gram of isopropanol alcohol (IPA) in a cylindrical, 20 cm³ stainless steel mill. 10 cm³ of 1 mm zirconia beads (YTZ® grinding media) are added to the mill. The mill is sealed and shaken to reduce the size of the R7 particles. During the process, the average particle size of the R7 particles is monitored by using a LA-300 Particle Size Analyzer. After the R7 particles have been reduced to the desired average particle size, a solution of 0.8 grams of CAB, 6.4 grams of toluene and 1.6 grams of IPA is added to the mill, and the mill is additionally shaken to homogenize the mixture. The mixture is then poured through a 0.3 mm sieve to remove the zirconia beads. For spray application, the viscosity of the mixture can be further reduced by adding toluene. The composition that results from the foregoing process can then be applied to a stent by using an EFD 780S spray coater.

Example 3

0.4 grams of dry R7 is combined with 0.2 grams of ethylene vinyl alcohol copolymer (EVAL) and 4 grams of dimethyl acetamide (DMAC) in a cylindrical, 20 cm³ stainless steel mill. 10 cm³ of 1 mm zirconia beads (YTZ® grinding media) are added to the mill. The mill is sealed and shaken to reduce the size of the R7 particles. During the process, the average particle size of the R7 particles is monitored by using a LA-300 Particle Size Analyzer. After the R7 particles have been reduced to the desired average particle size, a solution of 0.4 grams of EVAL and 8 grams of DMAC is added to the mill, and the mill is additionally shaken to homogenize the mixture. The mixture is then poured through a 0.3 mm sieve to remove the zirconia beads. For spray application, the viscosity of the mixture can be further reduced by adding DMAC. The composition that results from the foregoing process can then be applied to a stent by using an EFD 780S spray coater.

Example 4

0.4 grams of dry R7 is combined with 0.4 grams of PBMA and 4 grams of toluene in a cylindrical, 20 cm³ stainless steel mill. A probe from an ultrasonic homogenizer (Cole-Parmer Instrument Company, Vernon Hills, Ill.) is inserted into the mill and activated to reduce the size of the R7 particles. During the process, the average particle size of the R7 particles is monitored by using a LA-300 Particle Size Analyzer. After the R7 particles have been reduced to the desired average particle size of less than 2 microns, a solution of 0.8 grams of PBMA and 8 grams of toluene is added to the mill, and the mill is additionally shaken to homogenize the mixture. The mixture is then poured through a 0.3 mm sieve to remove the zirconia beads. For spray application, the viscosity of the mixture can be further reduced by adding toluene. The composition that results from the foregoing process can then be applied to a stent by using an EFD 780S spray coater.

Example 5

Dry R7 is placed in a Micron-Master® jet mill and the mill is activated. During the process, the average particle size of the R7 particles is monitored by using a LA-300 Particle Size Analyzer. After the R7 particles have been reduced to the desired average particle size, the R7 particles are dispersed in a solution of 1.2 grams of PBMA and 36 grams of toluene. The composition that results from the foregoing process can then be applied to a stent by using an EFD 780S spray coater.

Example 6

0.5 grams of R7 is dissolved in 10 grams of water. The R7 solution is introduced into an EFD 780S spray coater. The spray coater can be used to atomize the R7 solution. The atomized R7 solution is then sprayed into a pool of liquid nitrogen. The liquid nitrogen quickly freezes the R7 solution thereby producing R7-water particles. The liquid nitrogen is removed by slowly warming the liquid to about −5° C. The R7-water particles are then washed with acetone to remove the water. The R7 particles are dispersed into a solution of PBMA at a ratio of 1:2 (w/w) R7:PBMA and 1,4-dioxane at a ratio of 2:98 (w/w) of PBMA to 1,4-dioxane. The EFD 780S spray coater can be used to spray the drug-polymer suspension onto a stent.

Example 7

0.5 grams of R7 is dissolved in 10 grams of water. The R7 solution is introduced into an EFD 780S spray coater. The spray coater can be used to atomize the R7 solution. The atomized R7 solution is then sprayed into a pool of liquid nitrogen. The liquid nitrogen quickly freezes the R7 solution thereby producing R7-water particles. The liquid nitrogen is removed by slowly warming the liquid to about −5° C. The R7-water particles are then freeze-dried by feeding the particles into a FreeZone® Freeze Dry System (Labconco Corp., Kansas City, Mo.). The R7 particles are dispersed into a solution of PBMA at a ratio of 1:2 (w/w) R7:PBMA and 1,4-dioxane at a ratio of 2:98 (w/w) of PBMA to 1,4-dioxane. The EFD 780S spray coater can be used to spray the drug-polymer suspension onto a stent.

Example 8

0.5 grams of R7 is dissolved in 10 grams of water. The R7 solution is introduced into a Model BD-200 Bubble Dryer™ (Temco Instruments, Tulsa, Okla.). The dryer produces micronized R7 particles with an average particle size of about 0.65 microns which are then collected and dispersed into a solution of PBMA at a ratio of 1:2 (w/w) R7:PBMA and 1,4-dioxane at a ratio of 2:98 (w/w) of PBMA to 1,4-dioxane. An EFD 780S spray coater can be used to spray the drug-polymer suspension onto a stent.

While particular embodiments of the present invention have been shown and described, it will be obvious to those skilled in the art that changes and modifications can be made without departing from this invention in its broader aspects. Therefore, the appended claims are to encompass within their scope all such changes and modifications as fall within the true spirit and scope of this invention. 

What is claimed is:
 1. A method of coating a stent comprising: applying a composition to a stent; wherein the composition comprises: a micronized L-arginine polymer of formula: H—[NH—CHX—CO]_(p)—OH; wherein p is from 5 to 1,000 and X is —(CH₂)₃—NH—C(NH₂)═NH; and a fluid; and essentially removing the fluid from the composition on the stent to form a coating.
 2. The method of claim 1, wherein the micronized L-arginine polymer has an average particle size of less than 2 microns.
 3. The method of claim 1, wherein the composition comprises a second polymer, and wherein the second polymer dissolves in the fluid.
 4. The method of claim 3, wherein the solubility profile of the micronized L-arginine polymer is incompatible with the solubility profile of the second polymer.
 5. The method of claim 3, wherein the second polymer is a hydrophobic polymer.
 6. The method of claim 1, wherein the fluid is substantially an organic solvent.
 7. The method of claim 1, wherein the fluid is chloroform, acetone, dimethylsulfoxide, propylene glycol methyl ether, iso-propylalcohol, tetrahydrofuran, dimethylformamide, dimethyl acetamide, benzene, toluene, xylene, hexane, cyclohexane, heptane, octane, nonane, decane, decalin, ethyl acetate, butyl acetate, cyclohexanone, cyclohexanol, 1,4-dioxane, isobutyl acetate, isopropyl acetate, butanol, diacetone alcohol, benzyl alcohol, 2-butanone, dioxane, methylene chloride, carbon tetrachloride, tetrachloroethylene, tetrachloro ethane, chlorobenzene, 1,1,1-trichloroethane, formamide, hexafluoroisopropanol, 1,1,1-trifluoroethanol, hexamethyl phosphoramide or a combination thereof.
 8. The method of claim 1, wherein the micronized L-arginine polymer is L-heptaarginine, or an analog or derivative thereof.
 9. The method of claim 1, wherein the L-arginine polymer is micronized by a process including: providing a solution containing the L-arginine polymer; atomizing the solution to produce an aerosol; and directing the aerosol into a supercritical fluid to produce micronized particles.
 10. The method of claim 9, wherein the supercritical fluid comprises carbon dioxide, sulfur hexafluoride, a chlorofluorocarbon, a fluorocarbon, nitrous oxide, xenon, propane, n-pentane, ethanol, nitrogen, water, or a combination thereof.
 11. The method of claim 1, wherein the L-arginine polymer is micronized by a process including: providing a solution containing the L-arginine polymer; atomizing the solution to produce an aerosol; and directing the aerosol into a liquid gas to produce micronized particles.
 12. The method of claim 11, wherein the liquid gas comprises liquid nitrogen, liquid argon, liquid helium, liquid hydrogen, or liquid oxygen.
 13. The method of claim 1, wherein the micronized L-arginine polymer is produced by a process of wet milling or dry milling.
 14. The method of claim 1, wherein the composition further includes an active agent for inhibiting migration or proliferation of vascular smooth muscle cells. 